Frequency control



Nov. l, 1955 M. D. RUBIN FREQUENCY CONTROL 5 Sheets-Sheet 1 OriginalFiled May 25, 1948 /NVENTOR M/Lro/v D. RUB/N BY w 4' RNEY Nov. l, 1955M. D. RUBIN FREQUENCY CONTROL 3 Sheets-Shea t 2 Original Filed May 25,1948 /N VEN 'ro/2 M/L'ro/v D. RUB/N NOV- 1, 1955 M. D. RUBIN 2,722,607

FREQUENCY CONTROL Original Filed May 25, 1948 3 Sheets-Sheet 3 TIME 5F7C-5. 6 'CDU Ffzoug/vcv I Lb 5 O r- 5 Q be s 2 Q O U LU k) Q UJ u mm MS TIME E 9 h h g W MEMO@ A ORA/Ev United States Patent O FREQUENCYCONTROL Milton D. Rubin, Dorchester, Mass., assignor to RaytheonManufacturing Company, Newton, Mass., a corporation of Delaware Originalapplication May 25, 1948, Serial No. 29,004, now Patent No. 2,547,890,dated April 3, 1951. Divided and this application February 23, 1951,Serial No. 212,341

4 Claims. (Cl. 250-36) This application is a division of my copendingapplication, Serial No. 29,004, filed May 25, 1948, now Patent No.2,547,890, issued April 3, 1951.

This invention relates to electrical circuits, and more particularly toa pulse servo system suitable for frequency stabilization or automaticfrequency control of a generator.

An object of this invention is to devise an automatic frequency controlsystem which is rugged and reliable, and which is unaffected bymicrophonics, such as those generated by vibrations.

A further object is to devise a novel automatic frequency control systemfor a reex klystron oscillator.

A still further object is to provide a servo system having a sweeping orhunting characteristic such that, if the system is in a state in wlichno servo information is being generated, the system will sweep throughall possible states in succession until it does come into a region ofoperation in which servo information is generated, after which it willoscillate in a small neighborhood about the desired state.

An additional object is to devise a servo system in which the errordetector of the system need not have sense; that is, its output need nothave opposite polarities above and below its center value.

Yet another object is to devise a novel error detector for an automaticfrequency control servo system.

Still another object is to devise a novel clamping circuit arrangementwhich is useful in connection with a blocking oscillator.

Another object is to provide a sweep circuit utilizing a condenser whichis charged in a novel manner to provide a desired voltage variationthereon.

A further object is to devise a novel system for causing the voltage ona condenser to oscillate back and forth through a small range about adesired value.

The foregoing and other objects of the invention will be best understoodfrom the following description of an exemplication thereof, referencebeing had to the accompanying drawings, wherein:

Figs. 1 and 2 are curves useful in explaining the operation of theinvention;

Fig. 3 is a combined schematic and block diagram of a servo systemembodying the invention;

Fig. 4 is a circuit diagram of an embodiment of this invention; and

Figs. 5-9 represent waveforms occurring under various v conditions andat various points in the circuit of Fig. 4.

ln certain systems which use a local oscillator for mixing or for otherpurposes, it is desired that the output frequency of the said oscillatornot vary more than a small amount on either side of a predeterminedfrequency to which the oscillator has been adjusted. If the localoscillator is a reflex klystron, as disclosed illustratively herein,undesired variations or discrepancies in the output frequency may resultfrom changes in the internal dimensions of the klystrons resonantcavity, from changes in the resonator potential, or

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from other causes. The present invention provides an automatic frequencycontrol system for such a local oscillator.

For the purposes of the present invention, in which the local oscillatormay have a frequency on the order of 9,000 megacycles, for example, thefrequency used dictates the choice of a resonant cavity as asufficiently rugged, simple and reliable reference standard offrequency. A portion of the output signal of the oscillator, thefrequency of which is to be controlled, is passed through the referencecavity, and the rectified output of the cavity is examined. If thecavity is of the transmission type, the output as a function offrequency is, as represented by curve A in Fig. 1, a symmetricalresonance curve centered at the resonant frequency of the cavity. Insuch a case, the magnitude of the rectified output of the cavityindicates the degree of error from the cavity resonant frequency but notthe direction therefrom. However, if the frequency of the signal feedinginto the cavity is modulated slightly, the sign of the rectiedmodulation signal of the output is the same as that of the modulationsignal of the input if the carrier or rest frequency is in the region ofpositive slope of the resonance curve A, and vice versa if in thenegative slope region of said curve, thus indicating whether the signalis above or below the cavity resonant frequency. By this means, bothdirection and degree of error are indicated by the output of the cavity,and correction can be applied to the oscillator accordingly.

According to this invention, pulse frequency modulation is used insteadof sinusoidal or other type of continuous frequency modulation, in orderto eliminate the phase-determining or phase-comparison devices necessarywith such continuous frequency modulation. In Fig. l, consider a pulsefrequency modulation input signal B which is applied when the carrier orrest frequency is below the resonant frequency of curve A or in theregion of positive slope thereof. Such an input signal will produce arectified output modulation signal of the character shown at C, theamplitude of this signal being determined by projecting the upper andlower frequency limits of signal B to curve A to obtain correspondingoutput limits for output pulse C, and the length (in time) of pulsesignal C being equal to that of signal B. Under these conditions (whenthe carrier or rest frequency is below the resonant frequency), theoutput pulse C is of the same sign as the input pulse B.

Now consider a pulse frequency modulation input signal D which isapplied when the carrier or rest frequency is above the resonantfrequency of curve A or in the region of negative slope thereof. Such aninput signal will produce a rectied output modulation signal of thecharacter shown at E, the amplitude of this signal being determined byprojecting the upper and lower frequency limits of signal D to curve Ato obtain corresponding output limits for output pulse E, and the length(in time) of pulse signal E being equal to that of signal D. Under theseconditions (when the carrier or rest frequency is above the resonantfrequency), the output pulse E is of the opposite sign to input pulse D.

Thus, the direction of error is indicated by the relative polarity ofthe output pulse, the degree of error being indicated for theillustration described by the relative amplitude of the output pulse.

As illustrated by Fig. 2, there is some ambiguity when the rest orcarrier frequency is near the resonant frequency. Consider a pulsefrequency modulation input signal F which is applied at such a pointwith reference to the carrier or rest frequency as to span the resonantfrequency of curve A. Such an input signal will produce a rectifiedoutput modulation signal of the character shown at G, the amplitude andtime Width of this signal being determined as described hereinabove.Pulse output signal G contains both minus and plus portions, so that inthis situation there is some ambiguity as to the direction of error.However, the narrow positive portions of output signal G can besubstantially eliminated in succeeding circuits if the rise and falltimes of input pulse F are considerably less than the duration of thefiat top thereof; in this way, the ambiguity may be removed. It shouldbe noted that, even though the base line of input signal F is below thecavity resonant frequency, the output signal G is of opposite sign tothe input signal F, or is reversed with respect thereto. This means thatthe target frequency of the automatic frequency control system will beoffset from the resonant frequency of the reference cavity to a littlebelow said resonant frequency, since the target frequency of the systemaccording to the present invention is the frequency at which the outputsignal reverses with respect to lthc input signal. The amount of offsetcan be decreased by reducing the input pulse amplitude. As a matter offact, the amplitudes of the pulse input signals B, D and F are greatlyexaggerated in Figs. l and 2, as compared with the amplitudes of thepulse frequency modulation input signals actually used in the presentinvention.

1t is known to those skilled in the art that the output frequency of areflex klystron may be corrected or con trolled by appropriate changesin the repeller potential of such klystron. When a resonant cavity isused as a reference standard of frequency, regardless of what type oferror detector is used, there is a restricted lock-in range around theresonant frequency of the cavity, outside of which the rectified outputfrom the plumbing is too low to operate the servo system. Therefore, asweep circuit is incorporated so that, when the unit is turned on, theklystron output frequency will be swept automatically through theelectrical tunin" range of ;lystron until it is within lock-in range.

ln the servo system of the present invention, a staircase or stepwisesweep or frequency modulation or frequency variation of the klystron iscaused to occur, pulses for error detection and frequency control beingobtained from this sweep by differentiation of the rectified output ofthe reference cavity which is coupled to the klystron output.

Fig. 3 is a combined schematic and block diagram of a J.

servo system according to this invention. To generate the sweep voltagenecessary to vary the frequency of klystron 21 in steps on the order of500 kilocycles per step, a free-running step blocking oscillator il,running at a repetition rate on the order of 200,000 pulses per secuond, is employed. Condenser 2 is originally step-charged from blockingoscillator l, through a step rectifier 3, in a direction such that thepotential of the upper plate of said condenser becomes negative withrespect to the lower plate thereof, as represented by the steppedportion of the waveform in Fig. 5, which represents the staircasewaveform of the voltage of the upper plate of condenser 2 with respectto ground under sweeping or hunting conditions. By means of lead 4t, thevoltage on the upper plate of condenser 2 is applied to the repeller ofklystron Zll, so that Fig. 5 also represents the waveform of the voltageon the klystron repeller with respect to its cathode or ground undersweeping or hunting conditions. Since the step-charging of condenser 2is in a negative direction, the stepwise of pulse frequency modulationor frequency variation of the klystron 2i is from low to high frequency.

By way of illustration only, to vary the voltage across condenser 2 andon the klystron repeller by -18 volts. and to vary the klystronfrequency by approximately 60 megacycles, 120 steps are required fromthe step blocking oscillator il; the pulse output of said oscillator istherefore such that each step produces a change of 0.15 volt, which inturn causes the klystron frequency to change 500 kilocycles per step.

A portion of the output from the controlled klystron 21 is passedthrough a tuned or resonant cavity 75 having a frequency-output responsecharacteristic as indicated in Fig. 6. This characteristic has the sameshape as the curves A of Figs. l and 2 previously described, but isshown reversed and in the negative direction because this sort of aresponse characteristic is needed with the number of amplifier tubesused, for correct trigger polarity. Such a reversal of characteristicfrom those of Figs. 1 and 2 may be easily obtained by reversal of theconnections of the rectifier 76 to which the resonant cavity output isapplied.

When the klystron frequency is caused to vary in steps with time in themanner above described by means of step oscillator 1, the rectifiedoutput voltage of the cavity, whose characteristic is shown in Fig. 6,varies with time in the manner shown in Fig. 7, since the frequencysweep of the klystron is from low to high frequency. This voltage, thewaveform of which is shown in Fig. 7, is applied by means of lead 6 tothe input of an amplifier 7. The voltage waveform of Fig. 7 is amplifiedby amplifier 7, and is applied to an RC coupling circuit, including acondenser 8 and a resistor 9. This RC circuit 8 9 has a low timeconstant, such that said circuit acts as a differentiating circuit todifferentiate the signal applied to its input.

The differentiated waveform produced at the output of RC circuit 8 9 is,as shown in Fig. S, a series of pulses c and d which result from, andare produced simultaneously with, the risers a and b of the steppedvoltage wave of Fig. 7, which in turn result from, and are producedsimultaneously with, the intermittent or pulsed frequency changes ormodulations of the klystron 21 and the intermittent voltage changes ofcondenser 2. Throughout the present specification, when referring towaveforms in the shape of steps, terms commonly associated with steps orstairs will ordinarily be used; thus risers will be used to denote thevertical portions of such waveforms, while treads will be used to denotethe horizontal portions of such waveforms. it is only the changes orrisers e of the stepped voltage waveform of Fig; 5 which produce thechanges or risers a and b of the Fig. 7 waveform, since only the changesin the condenser voltage waveform produce changes in frequency of theklystron 21 and changes in the output of the reference resonant cavity;between such condenser voltage changes there is no change in klystronfrequency, consequently no change in output of the resonant cavity, andtherefore no pulses in the output of differentiating circuit 8 9. Theoutput of circuit 8 9, shown in Fig. 8, consists of pulses which reversein polarity just after the bottom f of the waveform of Fig. 7, since atsaid bottom the direction of the respective risers a and b thereof ineffect reverses. Comparing Figs. 7 and 8, it may be seen that there is areversal of polarity as between the two waveforms; that is, the positivepulses c of Fig. 8 correspond in time to the negative risers a of Fig.7. This is due to the fact that Fig. 7 represents the waveform of theinput to amplifier 7, and amplifier 7 inverts the waveform passingtherethrough; in other words, the direction of the stepped wave in theoutput of amplifier 7, and applied to circuit 8 9, would be the oppositeof that shown in Fig. 7, with initial positive voltage risers beingfollowed by negative risers The pulse waveform of Fig. 8, the output ofcircuit 3 9, is amplified and inverted by inverter 10 and then appliedto the grid of an automatic frequency control blocking oscillator 11.The grid of oscillator 11 is negatively biased, so that this oscillatoris a driven or oneshot blocking oscillator.

Since the pulse waveform of Fig. 8 is inverted by inverter 10, thewaveform applied to the grid of oscillator 11 consists of a series ofnegative pulses followed by positive pulses, the change in polarityoccurring after the bottom f of the Fig. 7 wave and just after thereach- S ing by the klystron 21 of the resonant frequency of thereference-tuned cavity 75, the characteristic of which is shown in Fig.6. When a positive pulse is applied to the grid of oscillator 11, saidoscillator is triggered to produce a high-voltage output pulse which isapplied across the primary 13 of an output transformer 14. The outputpulse of oscillator 11 is inverted through a secondary winding 15 oftransformer 14 and clipped by a rectifier 16 to bypass the negativebackswing thereof away from the rest of the circuit connected to winding15.

The output signal pulse of oscillator 11 is applied through a condenser17 and a rectifier 18 to charge condenser 2 in a direction opposed tothe charge placed thereon by oscillator 1, the amount of this oppositioncharge being determined by the voltage division between condensers 17and 2. In other words, the original negative charge on condenser 2 isreduced, thereby reducing the negative voltage with respect to groundwhich is applied to the klystron repeller, causing a lowering of theoutput frequency of the klystron 21. The voltage divisions from theoscillator 11 to condenser 2 and from the oscillator 1 to condenser 2are such that the former supplies an increment of charge to saidcondenser which is about three times as great as the latter.

After the pulse supplied from oscillator 11 to reduce the originalnegative charge on condenser 2 and to reduce the original negativevoltage applied to the klystron repeller ends, the klystron frequency isagain raised in steps as condenser 2 re-accumulates its originalnegative charge or as the voltage across said condenser is againincreased in the original negative direction by means of increments ofcharge supplied from step oscillator 1 through step rectifier 3. Thus,the klystron frequency oscillates back and forth through a smallneighborhood about the reference cavity resonant frequency. Fig. 9represents the voltage waveform on the klystron repeller with respect toits grounded cathode under the abovedescribed automatic frequencycontrol conditions. The downwardly-stepped portions g of the waveformrepresent the original periodic accumulations of negative charge oncondenser 2, or the original increases of voltage across said condenserin a negative direction, caused by the negative increments of chargesupplied to said condenser from oscillator 1; during the time of thesesteps, the klystron frequency is being increased in stepwise fashion.The substantially linear rising portions h of positive slope in thewaveform of Fig. 9 represent the periodic reductions of the originalnegative charge on condenser 2, or the increases of voltage across saidcondenser in a positive direction, caused by the positive pulsessupplied to said condenser from oscillator 11; during the time of eachof these linear portions the klystron frequency is being rapidly andsubstantially uniformly lowered. Since oscillator 11 supplies a slug ofcharge to condenser 2 which is about three times as great as thatsupplied thereto by oscillator 1, the change of repeller voltage causedby the linear portions h of the waveform in Fig. 9 is represented ascorresponding in amplitude or height to three steps g of the stepvoltage change produced by oscillator 1.

It has been stated previously that a sweep circuit is incorporated tosweep the klystron frequency automatically through the electrical tuningrange of the klystron Z1 until it is within the restricted lock-inrange; this sweep circuit is provided by the limit blocking oscillator19 acting with step oscillator 1. Oscillator 19 allows the klystronfrequency to hunt over a range of about megacycles, when it is notlocked in through the servoarnplilier 6-18. From Fig. 5, whichrepresents the staircase voltage waveform on the klystron repeller withrespect to ground or its cathode under these sweeping conditions, it maybe seen that, when a point 18 volts below the starting voltage isreached, the klystron repeller voltage as controlled by condenser 2 iscaused to return to the starting voltage, where it again starts to stepin a '6 negative direction or down to -18 volts'with respect t` thestarting voltage. Blocking oscillator 19 is so connected that a voltageof -18 volts across condenser 2 applied by lead 20 from the upper plateof condenser 2 to the blocking oscillator 19 will turn the tube ofoscillator 19 on; when such tube conducts, a blocked oscillation isgenerated by oscillator 19, in such a direction as to drive the voltageof condenser 2 by means of lead 20 back to the original starting voltageto start the cycle.

again. Thus, in Fig. 5, the stepped portions e of the waveform representthe original negative voltage stepping of condenser 2 by step oscillator1, while the Vertical portion k of said waveform represents the drivingof condenser 2 back to the starting voltage by a blocked oscillation oflimit oscillator 19. As long as the klystron frequency is locked inthrough the servoamplier 6-18, what might be called the reversing of theVoltage (as opposed to the original stepping of the Voltage byoscillator 1) is accomplished by oscillator 11 in the manner previouslydescribed in connection with Figs. 7-9; under these conditions thepotential of the upper plate of condenser 2 never becomes suiiicientlynegative with respect to that of its lower plate to turn on oscillator19, so that in the automatic frequency control condition oscillator 19remains olf. Oscillator 19 operates only when the unit is turned on, tobring the klystron frequency into lock-in range for servo operation, orin case the servoamplifier 6-18 fails to operate for some reason.

The above provides a somewhat general description of the operation ofthe system. Reference will now be made to Fig. 4 for a more detaileddescription of the circuit and its operation.

A reex klystron 21 is the oscillator the frequency of which is to bestabilized or controlled by the servo system of the present invention.Such a klystron may include a repeller electrode 22 which is connectedto lead 4 of the servoamplier in order to apply a proper potential tosaid repeller electrode, the potential applied to repeller 22 by meansof lead 4 being changed by the servoampliiier to change the outputfrequency of klystron 21. Said klystron also includes an emissivecathode 23 which is grounded, a control grid 24 biased slightly positivewith respect to said cathode by means of a battery 25, and an internalresonant output cavity 26 providing two grids in the tube through whichthe electrons pass, cavity 26 being biased positively a few hundredvolts with respect to cathode 23 by a battery 27 to provide anaccelerating voltage. Although the potential sources 25 and 27 areillustrated as separate batteries, this has been done only for purposesof simplification, the potentials for electrodes 24 and 26 preferablybeing obtained from the main potential source for the servoampliiier. Anoutput loop 28 is coupled to the internal cavity 26 in order to applythe output of klystron 21 to a coaxial transmission line 29. Saidtransmission line carries a portion of the klystron output t0 anintermediate frequency converter crystal (not shown) and the remainingportion of said output to a waveguide 30. Waveguide 30, together With acavity structure to be later described, provides a sampling and errordetecting device.

The step blocking oscillator 1 includes as the main element thereof apentode 31 having anode 32, suppressor grid 33, screen grid 34, controlgrid 35, and cathode 36. Anode 32 is connected through one winding 37 ofa pulse transformer 38 and through a resistor 39 to a source of positivepotential (not shown), on the order of volts. Suppressor grid 33 isconnected directly to grounded cathode 36, while screen grid 34 isconnected by way of leads 40 and 41 and resistor 39 to the positivepotential source, a bypass condenser 42 being provided between saidscreen grid and ground or cathode 36. Control grid 35 is connectedthrough a suitable resistor 43, winding 44 of pulse transformer 38, andanother resistor 45, to lead 41 and the positive potential source, tobias said control grid positively. A blocking condenser 46 is providedin the grid circuit of blocking oscillator 1, the lower plate of saidcondenser being connected to ground and the upper plate of saidcondenser being connected to the junction point of resistor and winding44.

The dots at windings 37 and 44 indicate similar polarities, and this isalso true at the other transformer windings in Fig. 4. For example, if acurrent ows through one winding so that the dot end is positive, thefield set up in the core induces voltages in the other windings, makingthe dot end positive in these windings at the same time.

By means of the above-described connections, with the polarities ofwindings 37 and 44 as indicated, with control grid 35 connected to apositive voltage, and with blocking condenser 46 connected as indicated,no trigger pulses being applied, pentode 31 is made to operate and willoperate as a free-running blocking oscillator at a rather' highrepetition rate, on the order of 200,000 pulses persecond, such pulsesor blocked oscillations being available at anode 32. As is understood bythose skilled in the art, the initial voltage swing at the anode 32,when tube 31 conducts, is in the negative direction, causing the voltagewith respect to ground to drop at said anode when tube 31 conducts.

In order to provide a charge on a condenser 47 before tube 31 conductsand during the intervals when it does conduct, this charge being suchthat the upper plate of said condenser has a positive potential withrespect to ground and the lower plate of said condenser has a negativepotential with respect to ground, the upper plate of said condenser,which is connected to anode 32, is also connected through winding 37 andresistor 39 to the positive potential source; the lower plate of saidcondenser is connected to anode 59 of a rectifier 5, the cathode 60 ofwhich is connected by means of lead 50 to tap 52 of a dual potentiometer51. In order to provide an adjustable potential negative with respect toground at tap 52, resistors 54, 53 and 56 are connected in series inthat order between ground 55 and a lead S7 which is connected to anegative potential source; tap or arm 52 engages and moves alongpotentiometric resistor 53. A resistor 61 is connected across rectier 5,while a bypass condenser 58 is connected between arm 50 or tap 52 andground.

Assuming for purposes of illustration that tap 52 is located at such apoint that the potential at said tap is negative 100 volts with respectto ground and that the potential of the positive source is 120 volts,the condenser 47 is charged to a voltage of 220 volts duringnon-conduction of tube 31 through a circuit traced as follows: positiveside of the positive source, resistor 39, winding 37, condenser 47,anode 59 and cathode 60 of rectier 5, negative tap 52 of potentiometer51, the positive side or ground of said potentiometer, and the groundedor negative side of the positive source. Since the voltage betweenpotentiometer tap 52 and ground and the voltage of the positive sourceare connected in series aiding in this circuit, the voltage acrosscondenser 47 is the sum of these two voltages and the upper platecondenser 47 has a potential of +120 volts with respect to ground andthe lower plate of said condenser has a potential of 100 volts withrespect to ground.

A parallel circuit to the one already described is provided between thelower plate of condenser 47 and tap S2, this circuit including inseries, from said plate of said condenser, the cathode 48 of rectiiier3, the anode 49 of said rectier, the main condenser 2, and lead 50.

When tube 31 conducts in accordance with the blocking oscillatoroperation of the circuit 1, its anode voltage with respect to ground orcathode 36 decreases substantially instantaneously, and for the purposesof the present discussion it may be assumed that this decrease amountsto 60 volts. At the instant of conduction in tube 31, its anode voltagewith respect to ground and also the voltage on the upper plate ofcondenser 47 with respect to ground drop to +60. volts from theiroriginal +120 volts. Since the charge on condenser 47 cannot changeinstantaneously, at said instant of conduction the lower plate ofcondenser 47 follows in potential the upper plate, also dropping 60volts and reaching a potential of -160 volts with respect to ground.This 160 volt absolute potential is more negative than the volt absolutepotential of tap 52', which means that the cathode 48 of rectiher 3 isnegative with respect to its anode 49 by some 60 volts, a conditionpermitting conduction in said rectifier. During the original pulse ofblocking oscillator 1, then, while tube 31 is conducting and while itsanode is +60 volts with respect to its cathode or ground, condenser 47partially discharges through tube 31, ground, resistor 54, part ofresistor 53, lead Si), condenser 2, and rectier 3, until the lower plateof condenser 47 reaches approximately the potential of tap 52, which is-l00 volts with respect to ground. When it does so, discharge ofcondenser 47 stops because at such time the cathode 48 of rectier 3 willno longer be negative with respect to its anode 49. During this time ofdischarge while tube 31 is conducting, the anode 59 of rectilier 5,connected to the lower plate of condenser 47, is negative with respectto its cathode, connected to lead 50, so that said rectifier cannotconduct. Throughout the present speci iication, the phrase absolutepotential or absolute voltage of an element means the potential orvoltage of such element with respect to ground.

During the time of this partial discharge of condenser 47 to a voltageof approximately 160 volts thereacross (its upper plate remaining at anabsolute potential of +60 volts and its lower plate going to an absolutepotential of approximately -100 volts), condenser 2 is charged, in sucha direction that its upper plate goes negative with respect to its lowerplate, to a small voltage, on the order of 0.15 volt, for example, thisvoltage change across condenser 2 being determined by the relationbetween the capacitances of condensers 47 and 2 and being much smallerthan the change of the voltage across condenser 47 because thecapacitance of condenser 2 is very large compared to the capacitance ofcondenser 47. Thus, a small increment of charge is supplied to condenser2 for or during each successive pulse of oscillator 1; in this way,condenser 2 is step charged with its upper plate negative with respectto its lower plate in response to the operation of oscillator 1.

When tube 31 is cut off because of the blocking action of condenser 46,this cutoff of said tube being substantially instantaneous, the absolutevoltage of anode 32 or the upper plate of condenser 47 instantaneouslyreturns to volts, the voltage of the positive potential source. Thus,the absolute potential of said upper plate rises 60 volts, and, sincethis change occurs substantially instantaneously and the charge oncondenser 47 cannot change instantaneously, the absolute potential ofthe lower plate of condenser 47 also rises 60 volts at this instant,from approximately -100 volts with respect to ground to approximately-40 volts with respect to ground. This approximately +40 volt absolutepotential is less negative or more positive than the -100 volt absolutepotential of tap 52, which means that the cathode 60 of rectifier 5 isnegative with respect to its anode 59 by some 60 volts, a conditionpermitting conduction in said rectifier. After the original pulse ofblocking oscillator 1, therefore, and between successive pulses,condenser 47 recharges through its original charging circuit tosubstantially its original value of charge, tube 31 being non-conductingor cut off during this time. Condenser 47 is recharged to a voltage ofsubstantially 220 volts thereacross, the absolute potential of its upperplate at the end of this charging period being +120 volts and theabsolute potential of its lower plate at this time being -100 volts.When condenser 47 is so charged, the charging thereof stops because atsuch time the cathode 60 of rectifier 5 will no longer be negative withrespect to its anode` SSL uring' this time of recharging of condenser 47while tube 31 is cut 01T or in the non-conducting condition, the cathode48 of rectifier 3, connected to the lower plate of condenser 47, ispositive with respect to its anode, connected to condenser 2 and lead50, so that said rectifier cannot conduct. Also, since condenser 2 ischarged during the pulses of blocking oscillator 1 in such a directionthat its upper plate becomes negative with respect to its lower plate,the potential applied to the cathode 48 of rectier 3 from the lowerplate of condenser 2 through resistor 61 is positive with respect to thevoltage applied directly from the upper plate of condenser 2 to theanode 49 of said rectifier, so that said rectifier cannot conduct todischarge condenser 2. Therefore, the increments of charge supplied tocondenser 2 during each pulse of oscillator 1 or during each time ofconduction of tube 31 are trapped thereon, so that condenser 2 ischarged in a stepwise manner by the operation of blocking oscillator 1,with its upper plate going negative with respect to its lower plate. Theupper plate of condenser 2 is connected, by means of leads 20 and 4,directly to repeller 22 of controlled klystron 21, in order to apply theabsolute potential of said upper plate or the voltage of said upperplate with respect to ground, as a controlling potential on repeller 22with respect to the grounded cathode 23, A predetermined bias voltage isprovided on repeller 22 through condenser 2 by lead 50 from tap 52,since the lower plate of said condenser is connected directly to lead50; the voltage at tap 52 is negative with respect to ground so that anegative bias with respect to grounded cathode 23 is provided onrepeller 22 by means of the above-described connection. An adjustable ypotential negative with respect to ground 55 or klystron cathode 23 isprovided through condenser 2 to repeller 22, to adjustably bias saidrepeller negatively with respect to cathode 23. Since the said biasvoltage is negative with respect to ground, and since condenser 2 isstep charged by the operation of oscillator 1 with its upper platebecoming negative with respect to its lower plate, the potential on theupper plate of condenser 2 with respect to ground has the steppedwaveform represented in Fig. 5, such potential being originally negativeand becoming more negative at steps e. The proper operation of a reexklystron requires that the repeller voltage with respect to the cathodebe negative at all times. The step increases e in the negative directionof the absolute potential on the upper plate of condenser 2, suchabsolute potential being applied to repeller 22, cause the outputfrequency of klystron 21 to sweep in a stepwise manner from a low to ahigh value.

It is advantageous to provide a linear staircase charging of condenser 2by the operation of oscillator 1, to make all the risers e in Fig. 5 ofsubstantially equal height. This linear charging, rather thanexponential charging, is desirable, and may be accomplished by applyingto the condenser constant current pulses rather than constant voltagepulses, since the latter provide eX- ponential charging of saidcondenser because of the opposing voltage across said condenser as thecharge on the condenser increases. This invention contemplates andaccomplishes staircase charging of condenser 2 or staircase variation ofthe voltage across said condenser from a source of substantiallyconstant current pulses. Tube 31 is a pentode and therefore has platecharacteristics such that, for each value of control grid voltage, theplate voltage-plate current curve has a portion of substantiallyconstant positive slope and a portion of substantially zero slope. Thelatter portion is known as the constant current portion of the pentodecharacteristic.

According to this invention, blocking oscillator pentode 31 is sooperated as to in effect supply constant current increments to condenser2 each time said tube conducts by means of the discharge circuit forcondenser 47 as previously described. When tube 31 generates a blockedoscillation, grid current flows therein because4 the grid 35 .of saidtube is driven positive by th'e .plate voltage change. The resistance ofresistor 43 is large compared to the grid-cathode resistance ofapproximately 1,000 ohms which exists when grid current is drawn inalmost all vacuum tubes. The resistor 43 and the grid-cathode resistanceof tube 31 provide a voltage divider. Since resistor 43 has a ratherhigh resistance, the ow of grid current in tube 31 is limited, thuslimiting the positive voltage on the grid 35 with respect to groundduring conduction in tube 31. The said positive grid voltage is limitedto a value such that pentode 31 is limited to a constant current regionfor the plate voltage thereon and does not reach the region of positiveslope in its plate characteristic grid-voltage family for said platevoltage, which it would do if the positive absolute voltage on the gridwere not so limited but were allowed to increase. By so limiting pentode31 to its constant current region during conduction, it in effectprovides a constant current discharge circuit for condenser 47 each timeit conducts, thus providing increments of constant current to condenser2 to produce linear staircase or stepwise charging of condenser 2.

/The limit blocking oscillator 19 will next be described. Oscillator 19includes, as the main element thereof, a triode 62 having anode 63, grid64, and cathode 65. Cathode 65 is connected directly to the upper plateof main condenser 2, so that the potential on this plate is applied atall times directly to said cathode. Grid 64 is, connected through onewinding 66 of a pulse transformer 67 to a lead 68 which is in turnconnected to a movable arm or tap 69 of potentiometer 51. Arms 52 and 69are mechanically coupled together, as indicated, so that they movetogether. The potentiometric resistor 70 which arm 69 engages has oneend connected through resistor 71 to ground 55 and its opposite endconnected to lead 57, which is connected to a negative potential source.In this way, a negative bias potential with respect to ground is appliedto control grid 64, said grid of tube 62 being biased to a point thatrequires a relatively large negative voltage with respect to groundapplied to cathode 65 to turn tube 62 on. A bypass condenser 72 isconnected between leads 50 and 68.

Anode 63 is connected through a winding 73 of transformer 67 to thelower plates of condenser 2, a small condenser 74 being connected acrosswinding 73. The voltage across condenser 2 is therefore theanode-cathode voltage of tube 62. The h18 volts applied to cathode 65necessary to turn tube 62 on means that, when condenser 2 is charged to18 volts so that its upper plate or the cathode 65 has a potential of-18 volts with respect to its lower plate or the anode 63, tube 62 willconduct, but when condenser 2 is charged to less than --18 volts, tube62 will not conduct. Taps 52 and 69 of the dual potentiometer 51 aremechanically coupled together in such a way that the potential of tap 69applied to grid 64 will at all times, throughout the range of movementof said taps for variationof the bias on repeller 22, be negative, withrespect to that applied by tap 52 to the lower plate of condenser 2 andsaid repeller, by an amount just slightly less than the algebraic sum ofthe -18 volt ligure selected and the cutoff voltage of tube 62 with 18volts applied between the anode 63 and the cathode 65. In this way, itis assured that, when a potential of 18 volts across condenser 2 isreached, during the charging thereof by the operation of oscillator 1,tube 62 will conduct.

Condenser 2 is, in eiiect, the blocking condenser for blockingoscillator 19, this condenser being in the cathode circuit of this limitblocking oscillator and serving as a cathode blocking condenser, ratherthan this oscillator being provided with a grid blocking condenser as inother blocking oscillators. When tube 62 conducts, at the end of thesweep, or when the voltage across condenser 2 reaches 18 volts, there isan excess of electrons on the upper plate of condenser 2, since at thistime' the upper plate of said condenser is at a potential of. .18 voltsrelative to the lower plate thereof. When tube 62 conducts, electronsare drawn oi the upper plate of condenser 2 by drawing them from thecathode 65 which is connected to said upper plate, causing the upperplate of condenser 2 to go back in a positive direction, dischargingsaid condenser. Thus, the negative voltage sweep or charging ofcondenser 2 is ended, permitting step oscillator 1 to begin its nextcycle of stepwise charging of condenser 2, and at the same time cuttingoff tube 62 by dropping its anode-cathode voltage to substantially zero.Oscillator 19 is thus a driven or one-shot blocking oscillator.

Referring again to Fig. 5, the stepwise increasing in a negativedirection of the absolute voltage on the upper plate of condenser 2 isproduced by the charging of said condenser by the operation of stepblocking oscillator 1, while the substantially vertical increase, in apositive direction, of said absolute upper plate voltage as at k (whenthe voltage across the condenser has reached 18 volts) is produced bythe discharging of said condenser 2, by limit blocking oscillator 19, inthe manner just described. Tube 62 remains cut off, and blockingoscillator 19 remains blocked, until the voltage across condenser 2again reaches 18 volts at the end of the next staircase sweep of saidvoltage by the operation of step oscillator 1, at which time tube 62again conducts because of the sufficiency of the anode-cathode voltagethereon, the grid bias permitting conduction under these voltageconditions; when tube 62 conducts, condenser 2 again is discharged.

These voltage variations during the hunting or sweeping condition causestaircase-sweep frequency modulation of the klystron output frequencyfrom low to high frequency, followed by almost instantaneous return ofsaid klystron output frequency from high to low frequency.

Waveguide 30 has coupled thereto, intermediate its ends, a resonantcavity 75 which may, for example, be a circular cylindrical cavityresonating in the TE1,1,1, mode. Said cavity serves as a referencestandard of frequency, providing a bandpass filter having an outputvoltage-frequency response characteristic of the slope generally shownin Fig. 6, which is similar to the conventional curve for a singlyresonant circuit. A crystal detector 76 is also coupled to waveguide 30,at the output side of the filter 75, the pilot signal from localoscillator 21 being fed into the input of filter 75 by transmission line29 and waveguide 30, as previously described.

The rectified modulation signal of the output of the waveguide andcavity structure appears at the output leads 6 of crystal detector 76.When the klystron output frequency is caused to vary in steps by theoperation of step oscillator 1 in thet manner previously described, thecurve of Fig. 6 appears on a time scale as shown in Fig. 7. The waveformcurve of Fig. 7 represents, therefore, the waveform of the voltageproduced between leads 6.

One of the leads 6 is connected to ground 55, while the other isconnected through a coupling condenser 77 to the control grid 78 of thepentode 79 which serves as the amplifier stage 7. A leak resistor 80 isconnected between control grid 78 and grounded cathode 81 of pentode 79.Suppressor grid 82 of tube 79 is connected to cathode S1. Screen grid 83of said tube is connected through a resistor 84 to a source of positivepotential on the order of 120 volts, a bypass condenser 85 beingconnected between grid 83 and ground 55 or cathode 81'. Anode 86 of tube79 is connected through a pair of resistors 87 and 8S to a source ofpositive potential, not shown, on the order of 120 volts. By means ofthe aforesaid connections, pentode 79 operates as an amplifier toamplify the voltage wave of Fig. 7, which is the input to saidamplifier, tube 79 also functioning to invert the input signal thereto.

In order to couple the output of amplifier 7 to the inverter 10, anode86V is connected through a coupling condenser 8 having a low capacitanceto the control grid 89 of an amplifier and inverter pentode 90, thecoupling circuit being completed by a leak resistor 9 connected betweengrid 89 and ground 55. The RC coupling circuit 8-9 has a lowtime-constant, such that said circuit acts to differentiate the signalapplied to its input, which signal is the output of amplifier 7. Thedifferentiated waveform, the output of coupling circuit 8 9 or the inputsignal of amplifier and inverter stage 10, has the shape shown in Fig.8, it being remembered that the input signal to circuit 8 9 has awaveform of the same character as that shown in Fig. 7 but inverted withrespect thereto. Due to the differentiating action of circuit 8 9, asteep impulse is produced in the output of said circuit simultaneouslywith each of the risers a and b in the stepped waveform of Fig. 7, sinceduring these risers the time rate of change of the input voltage ofcircuit 8 9 is very large, while during the treads of the steps the timerate of change of the input voltage of said circuit is substantiallyzero. The output pulses of circuit 8 9 therefore coincide in time withcorresponding risers in the steps of Fig. 7. The initial pulses c ofFig. 8 are in a positive direction, since the initial risers in theinput wave to circuit 8 9 are in a positive direction, the inverse ofFig. 7. The polarity of the output pulses of circuit 8 9 reverses at orimmediately after the time of the bottom f of the Fig. 7 wave, since atthis time the direction of the risers of Fig. 7 reverses, makingnegative risers at the input of circuit 8 9 and consequent negativepulses d at the output of said circuit simultaneously with such negativerisers.

From a comparison of Figs. 6 and 7, it may be seen that the bottom f ofthe wave of Fig. 7, or the reversal of the direction of the risers a andb thereof, occurs when the resonant frequency of the reference cavity 75is reached by the step-frequency-modulated klystron 21. Since this isso, the reversal of polarity of the output pulses of circuit 8 9 occurssimultaneously with the reaching of the resonant frequency of cavity 75by the klystron 21. Thus, pulses for error detection are obtained bydifferentiation of the rectified output of cavity 75, the polarity ofthese pulses indicating the direction of error from the target frequencyor resonant frequency of reference cavity 75.

The servo amplifier is inherently low-microphonic. Because of the lowtime constant of circuit 8 9 in the grid circuit of the second amplifiertube 90, frequencies in the microphonic range are considerablyattenuated.

The pentode 90 of the amplifier and inverter stage 10 includes, inaddition to control grid 89, an anode 91, a suppressor grid 92 connectedto grounded cathode 93, and a screen grid 94 connected to the positivepotential source through resistor 84. Anode 91 is connected through aresistor 95 to resistor 88 and the source of positive plate potential. Abypass condenser 96 is connected between the junction of resistors 88and 95 and ground S5. Pentode L90 functions to amplify and invert theinput pulses c and d of Fig. 8 applied thereto, to provide in the outputof said pentode negative impulses followed by positive impulses, thepositive impulses occurring after the resonant frequency of referencecavity 75 has been reached.

In order to couple the output of amplier and inverter stage 10 to theautomatic frequency control blocking oscillator 11, anode 91 isconnected through a coupling condenser 97 to the control grid 9S of atriode 99 which constitutes the heart of the driven or one-shot blockingoscillator 11. In order to bias grid 98 negatively with respect togrounded cathode 107, to provide the desired one-shot action for tube99, a pair of resistors 101 and 102 is connected in series betweenground 55 and negative lead 57 to provide a voltage divider; the commonjunction of these two resistors is connected through a grid resistor togrid 98. A bypass condenser 103 is connected across resistor 101, theinterwinding capacity of transformer 14 providing the blockingcapacitance for blocking oscillator 11. Anode 104 of triode 99 isconnected through a winding 13 of block-oscillator transformer 14 andthrough a resistor 105 to the positive potential source, a bypasscondenser 106 being connected between ground and the junction betweenresistor 105 and winding 13.

Blocking oscillator triode 99 is connected so that the feed-back windingof transformer 14 is in the cathode circuit, the grid 98 not beingconnected directly to the transformer; to carry out this purpose,cathode 107 is connected through winding 108 of transformer 14 to groundat 109. Thus, during the trigger period, before tube 99 goes into ablocked oscillation, the grid 98 has a suiciently high negative bias tocut off the tube and thus the grid circuit presents a high impedance tothe previous plate 91, resistor 100 having a high impedance, permittingthe use of high enough plate impedance in tube 90 to developconsiderable amplification. Therefore, blocking oscillator 11 can betriggered directly from a high-impedance source without a trigger tube.

Transformer 14 is a special blocking oscillatortransformer particularlyadapted to this type of operation. The turns ratio from plate to cathodeto output winding 15 is 3:l:1.5, and approximately the same amplitudepulse is developed across each winding because of capacitive couplingand possibly because of saturation effects. The coils are wound forminimum capacity to each other.

A clamping diode rectifier 12 has its cathode 110 connected to grid 98and its anode 111 connected to the ungrounded side of resistor 101. Grid98 is negatively biased, as stated above, so that tube 99 is normallyoff or non-conducting. When the grid 98 is triggered in a positivedirection by a positive pulse appearing in the output of stage 10, tube99 conducts, the voltage of plate 104 goes in the negative direction,and a voltage is induced in cathode winding 108 driving cathode 107negative, which is in a regenerative direction.

Without rectifier 12, grid 98 is tied to a high-impedance circuit, sinceresistor 100 has a high impedance. The grid-cathode resistance of tube99, when the cathode 107 becomes negative and grid current is drawn, islow, and the circuit impedance of the cathode is low. Thus, the grid 98is driven by the cathode circuit, and becomes almost as negative as thecathode. Because the grid 98 is then not very positive with respect tothe cathode 107, the plate current through tube 99 is not very high, andthe output pulse at output transformer winding 15 is unsatisfactory involtage amplitude. i

However, with rectifier 12 connected as disclosed, grid 98 is no longertied to a high-impedance circuit when grid current is drawn, since whengrid current flows the electron flow is from cathode 107 to grid 98,cathode 110, anode 111, to ground and back to cathode 107. Rectifier 12will conduct when electron ow is in this direction, and the impedance ofsaid rectifier is very low when the same conducts, providing thereforesubstantially a short circuit across resistor 100; as a result, underthese conditions grid 98 has a low external circuit impedance when gridcurrent is drawn. Due to this low external circuit impedance, grid 98 isclamped by rectifier 12. Thus, large grid current can flow and grid 98is no longer driven by the cathode circuit but can remain Very positivewith respect to cathode 107 as said cathode is driven negative. Heavypulse current then ows in tube 99 as a result of this large positivegrid voltage with respect to the cathode, approximately 80 volts beingdeveloped across the load with a 1Z0-volt plate supply during theblocked oscillation generated.

Tube 99 is operated near cutoff so that only the positive-going portionof the pulse output signal of stage causes said tube to be triggered. Asdescribed above, the positive pulses occur after the resonant frequencyof reference cavity 75 has been reached. The additional the charge oncondenser 2, the voltage on repeller .22,l

and the output frequency of klystron 21, this variation of klystronoutput frequency resulting in a positive pulse to trigger oscillator 11if the klystron output frequency is above the resonant frequency ofreference cavity 75, and in a negative pulse of the klystron outputfrequency is below said resonant frequency.

Secondary winding 15 of blocking oscillator transformer 14 is connectedas indicated by the dots adjacent windings 13 and 15. The upper end ofwinding 15 is connected through a condenser 17 to the anode 112 of adiode rectifier 18 the cathode 113 of which is connected to the lead 20and the upper plate of main condenser 2. The lower end of winding 15 isconnected to negative potential lead 68. A diode rectifier 16 has itscathode 114 connected to the common junction of condenser 17 and anode112, and its anode 115 connected to lead 68 and the lower end of winding15. A resistor 116vis connected across rectifier 16. Rectifier 16functions as a clipper to remove or bypass the negative backswing of`the output pulse of oscillator 11, which appears across winding 15,`away from the remainder of the output circuit including rectifier 18and condenser 2, and also to discharge condenser 17 after a pulse ofoscillator 11, since after such pulse the left-hand plate of saidcondenser, or anode 115, is positive with respect to the righthand plateof said condenser, or cathode 114, and since after such pulse the lowerend of winding 15 returns to the absolute potential established by tap69. Here, the term negative means that the upper end of winding 15 isnegative with respect to its lower end. Under these conditions, cathode114 of rectifier 16 is negative with respect to its anode 115, and saidrectifier conducts.

As previously stated, the potential of tap 69 is at all times negativewith respect to that of tap 52. Lead 68, connected to tap 69, thereforeapplies a potential, through resistor 116 to the anode 112 of rectifier18, which is negative with respect to that applied by lead 50 from tap52 through condenser 2 to the cathode 113 of said rectifier. Thus,rectifier 18 is biased with its anode negative with respect to itscathode, so that it will present a high-resistance direct current pathto the step charge on main condenser 2, in which the upper plate ofcondenser and cathode 113 are negative with respect to the lower plateof said condenser. t

Condensers 17 and 2 function as a capacitance-type voltage divider. Thevoltage of the pulse developed across the load by oscillator 11 issufficient to overcome the bias on rectifier 18 and charge condenser 2in a direction the reverse of the step charge by an amount determinedbyl the voltage division between Condensers 17 and 2. In other words,the output pulse generated by blocking oscillator 11 when it istriggered is applied through condenser 17 and rectifier 18 to reduce thestep charge on main capacitor 2, this step charge having been placedthereon by the action of step blocking oscillator 1 in the mannerpreviously described. This reduction of the step charge on condenser 2reduces the negative voltage applied to the klystron repeller 22 andcauses the output frequency of the klystron 21 to be lowered. Thecircuit from the winding 15, across which the generated pulse appears,to condenser 2 may be traced as follows: upper end of winding 15,condenser 17, anode 112 and cathode 113 of rectier 18, condenser 2, lead50, condenser 72, and lead 68, to the lower end of winding 15, thecondenser 72 normally being charged to a-voltage 15 equal to thepotential difference between leads 50 and 68.

The voltage division from the automatic frequency control blockingoscillator 11 to condenser 2 and that from the step blocking oscillator1 to condenser 2 are such that the former supplies a slug of chargeabout three times as great as the latter. When the pulse fromtransformer 14 ends, the klystron frequency is again raised in steps ascondenser 2 accumulates a step charge from oscillator 1 throughrectifier 3. Since in the stepwise sweep of the charge on the upperplate of condenser 2 negatively with respect to ground or the staircasesweep of the klystron 21 from low to high frequency by step oscillator1, positive pulses are produced in the output of circuit 8 9 after theresonant frequency of reference cavity 7S has been reached andsimultaneously with the frequency changes of klystron 21, since positivepulses trigger the automatic frequency control blocking oscillator 11 toreduce the step charge on condenser 2, and since oscillator 11 suppliesa slug of charge to condenser 2 about three times as great as thatsupplied thereto by oscillator 1, the waveform of the voltage withrespect to ground on the klystron repeller is as shown in Fig. 9, andthe klystron frequency oscillates about the resonant frequency of cavity75. In other words, when the servoamplifier 6-18 is in operation or whenthe system is in the automatic frequency control condition, the waveformof the voltage with respect to ground on klystron repeller 22 is asshown in Fig. 9, in which the downwardly-stepped portions g representthe periodic accumulations of charge on condenser 2 produced by theoperation of step oscillator 1 and in which the substantially linearrising portions h of positive slope represent the periodic reductions ofcharge on said condenser caused by the supply of corresponding positivepulses to said condenser from oscillator 11. During the time of thesteps g in Fig. 9, the klystron frequency is being increased in stepwisefashion, while during the time of the positively-sloping linear risingportions h the klystron frequency is lowered.

The limit blocking oscillator 19 operates in the manner previouslydescribed to cause hunting of the absolute potential of the upper plateof condenser 2 and of the klystron frequency over a rather wide rangewhich includes the lock-in range of the servoamplifier 6-18, when theklystron frequency is not locked in through the servoamplier for somereason.

The servo system of this invention not only supplies error informationin the form of pulses, as illustrated in Fig. 8, but correction is bymeans of pulses. The two amplifier stages 7 and 10 feed blockingoscillator 11 and,

as long as the positive pulse at the input to oscillator 11 issufficient to trigger the same, the system is insensitive to any furtherchange in amplitude, whether it be due to degree of error or tomodulation of the signal at microphonic frequency.

The mechanism of frequency correction is as follows. If the klystronoutput frequency is below the resonant frequency of reference cavity 75,the staircase sweep drives the klystron frequency in the correctdirection. Correction takes place at about 500 kilocycles perfivemicrosecond step, or one megacycle in ten microseconds. If theklystron frequency is above the resonant frequency of the referencecavity, but within lock-in range, then for each slug of charge from theaction of step blocking oscillator 1 into condenser 2, driving theklystron frequency one step higher, the automatic frequency controlblocking oscillator 11 charges condenser 2 three times as much in theopposite direction, since for each such slug from oscillator 1 apositive pulse is produced which triggers oscillator 11. Thus, with eachpulse of the step blocking oscillator, the klystron frequency moves inthe correct direction (3 1) 500:1000 kc., until it is back in theneighborhood of the correct frequency. Correction thus takes place atabout 1000 kilocycles per tive-microsecond step, or one megacycle infive microseconds. If for some reason the klystron frequency is abovethe resonant frequency but outside lock-in range, it will step to thehighest frequency, where the limit blocking oscillator 19 triggers off,and the klystron frequency returns to the lowest value, whence it stepsto the correct frequency, all within about 600 microseconds.

The high frequency of operation of this system makes it possible tocorrect satisfactorily for frequency modulation due to vibration of theklystron elements or due to power supply ripple. It also reduces theeffects of leakage from condenser 2, and makes it possible to use asmaller condenser in this location. In fact, one of the main advantagesof my invention is that there are not very many condensers of largecapacity used in the entire system.

Due to the use of a differentiating circuit in the present system, whichproduces pulses of opposite polarities above and below the resonantfrequency of system, the output of the error detector itself, which isillustrated in Fig. 7, need not have sense.

Of course, it is to be understood that this invention is not limited tothe particular details as described above, as many equivalents willsuggest themselves to those skilled in the art. For example, since theservo system of this invention is well adapted for frequencystabilization, it can control frequency by controlling a reactance orresistance tube, or by controlling voltage in any system of which thefrequency is a function of a voltage. The control of the outputfrequency of a klystron oscillator by control of its repeller voltagehas been described herein merely by way of example. The system of thisinvention can beused to control other quantities than frequency if theerror detector response has the form of a resonance curve. Various othervariations will suggest themselves. It is accordingly desired that theappended claims be given a broad interpretation commensurate with thescope of this invention within the art.

What is claimed is:

1. An error detector for a frequency stabilizing system comprising: anoscillator for generating electrical oscillations of a predeterminedfrequency; tuned means coupled to said oscillator and having a responsecharacteristic symmetrical with respect to said predetermined frequency;means for frequency modulating said oscillator in a series of relativelyminor additive steps followed by a single relatively major subtractivestep traversing a frequency band including said predetermined frequency;and means for rectifying the output of said tuned means to obtain acontrol voltage the amplitude and polarity of which are functions,respectively, of the magnitude and direction of the deviation of thefrequency of said oscillator from said predetermined frequency.

2. An error detector for a frequency stabilizing system comprising: anoscillator for generating electrical oscillations of a predeterminedfrequency in the microwave region of the electromagnetic spectrum; acavity resonator coupled to said oscillator and tuned to saidpredetermined frequency; said cavity resonator having a respensecharacteristic symmetrical with respect to said predetermined frequency;means for frequency modulating said oscillator in a series of additivesteps followed by a single subtractive step traversing a frequency bandincluding said predetermined frequency; and means for rectifying theoutput of said cavity resonator to obtain a control voltage theamplitude and polarity of which are functions, respectively, of themagnitude and direction of the deviation of the frequency of saidoscillator from said predetermined frequency.

3. An error detector for a frequency stabilizing system comprising: anoscillator for generating electrical oscillations of a predeterminedfrequency; tuned means coupled to said oscillator and having a responsecharacteristic symmetrical with respect to said predetermined frequency;a source of pulses coupled to said oscillator fof frequency f modulatingsaid oscillator in steps of small magnitude in the additive directionand of larger magnitude in the subtractive direction, all of said stepsbeing within a narrow band including said predetermined frequency; andmeans for rectifying the output of said tuned means to obtain a steppedcontrol voltage the instantaneous amplitude and polarity of which arefunctions, respectively, of the magnltude and direction of the deviationof the frequency of said oscillator trom said predetermined frequency.

4. An error detector for a frequency stabilizing system comprising: anoscillator for generating electrical oscillations of a predeterminedfrequency in the microwave region of the electromagnetic spectrum; acavity resonator coupled to said oscillator and tuned to saidpredetermined frequency; said cavity resonator having a responsecharacteristic symmetrical with respect to said predetermined frequency;a source of pulses coupled to said oscillator for frequency modulatingsaid oscillator in steps of small magnitude in one direction and oflarger magnitude in the opposite direction, all of said steps beingwithin a band including said predetermined frequency; and means forrectifying the output of said cavity resonator to obtain a steppedcontrol voltage the instantaneous amplitude and polarity of which arefunctions, respectively, of the magnitude and direction of the deviationof the frequency of said oscillator from said predetermined frequency.

References Cited in the le of this patent UNITED STATES PATENTS2,404,568 Dow July 23, 1946 2,462,294 Thompson Feb. 22, 1949 2,475,074Bradley July 5, 1949 p 2,510,095 Frankel June 6, 1950 2,583,023Spangenberg n Ian. 22, 1952 2,640,156 Schultz May 26, 1953 2,640,157Wallman May 26, 1953

